Picture signals including those of the NTSC, PAL and SECAM type all have a chroma and a luminance signal which comprises the picture information. The chroma signal bandwidth is narrow in comparison with the luminance signal bandwidth. Because of the limited frequency bandwidth of the chroma signal, chroma signal transitions (transients) are relatively slow. That is to say, the slope of a transition representing color edges has only a moderate slope, which when displayed for viewing detracts from sharp color demarcations. For instance, the transition of a typical chroma signal from the color green to the color magenta is not very steep, causing a smeared color to be viewed at the color edges.
Television systems with digital signal processing are distinguished in that the chrominance signal includes color information for discrete pixels. Digital signal processing is typically performed on these digital chrominance signals to enhance the sharpness of color transitions, especially between adjacent pixels. One such technique is disclosed in U.S. Pat. No. 4,935,806 to Rabii entitled "Chroma Noise Reduction and Transient Improvement". This technique provides an enhanced transient chroma signal with controlled undershoot and overshoot characteristics. However, this technique is somewhat complex.
Another known technique is shown in FIG. 1, whereby two bandwidth optimized paths are implemented in order to detect the position of a color transition in the incoming chrominance signals. The exact position of a color transition is calculated by detecting the corresponding zero transition of the second derivative of both chrominance signals. Low pass filtering (LPU,LPV,LPUV) is performed to avoid noise sensitivity. The width of a transition is derived from a threshold detector signal. It indicates an area around the detected position where the first derivatives of the chrominance signals exceed a programmable threshold (THRESH). The parameter THRESH modifies the sensitivity of the DCTI circuit. High THRESH values exclusively improve significant color transitions while small color variations remain unchanged. The detected transition width can be limited by the programmable parameter TRAWID. This parameter performs an adaption to the input chrominance bandwidth. For signals with small chrominance bandwidth (e.g. Video Recorders) the DCTI performance is optimized using high values for TRAWID. Input signals with high chrominance bandwidth are processed with small values of TRAWID. A more detailed discussion of this approach is disclosed in an article published in The Journal IEEE Transition on Consumer Electronics, August 1993, Volume 39, Page 247-254 entitled, A Digital Display Processor with Integrated 9 Bit Triple DAC for Enhanced TV Applications, authored by Siemens. Again, this approach is rather complex and an expensive implementation.
It is desired to provide an improved digital color transition improvement (DCTI) algorithm whereby a chrominance signal can be processed to have a steep chroma edge without ringing. The improved algorithm should be a simple architecture with logical operation, and well considered for the implementation by digital signal processing. Preferably, the algorithm could be implemented by an Scan-Line Video Processor (SVP) with few instructions, or implemented by a small number of gates on an ASIC or FPGA.